Process for treating polyolefin discharged from an olefin polymerization reactor which comprises steps of discharging from a polymerisation reactor a slurry comprising polyolefin and diluent; submitting said slurry to a pressure release such that the diluent is evaporated and a polyolefin/gas mixture is formed; discharging said polyolefin/gas mixture into a collecting vessel; controlling a level of settled polyolefin in the collecting vessel; discontinuously transferring the polyolefin/gas mixture in the collecting vessel into a concentrating vessel; and discharging said concentrator vessel in such a way that said polyolefin/gas mixture is transferred into the hydrocarbon purge bin.Embodiments also comprise the use of two interconnected concentrator vessels and the optimized recovery of the diluent from the purge bin.